pubs.acs.org/Langmuir © 2010 American Chemical Society
Double-Sided Anodic Titania Nanotube Arrays: A Lopsided Growth Process Lidong Sun,† Sam Zhang,*,† Xiao Wei Sun,‡ Xiaoyan Wang,† and Yanli Cai† †
School of Mechanical and Aerospace Engineering and ‡School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received August 25, 2010. Revised Manuscript Received October 5, 2010
In the past decade, the pore diameter of anodic titania nanotubes was reported to be influenced by a number of factors in organic electrolyte, for example, applied potential, working distance, water content, and temperature. All these were closely related to potential drop in the organic electorlyte. In this work, the essential role of electric field originating from the potential drop was directly revealed for the first time using a simple two-electrode anodizing method. Anodic titania nanotube arrays were grown simultaneously at both sides of a titanium foil, with tube length being longer at the front side than that at the back side. This lopsided growth was attributed to the higher ionic flux induced by electric field at the front side. Accordingly, the nanotube length was further tailored to be comparable at both sides by modulating the electric field. These results are promising to be used in parallel configuration dye-sensitized solar cells, water splitting, and gas sensors, as a result of high surface area produced by the double-sided architecture.
Introduction In 1999, anodic columnar porous film was formed on titanium in fluorinated electrolyte by Zwilling and co-workers.1 Two years later, an anodic titania nanotube array was first reported by Grimes and co-workers.2 Thereafter, it has been vigorously investigated, profiting from its high surface area and highly oriented alignment, and extensively explored in versatile applications, such as dyesensitized solar cells,3-5 hydrogen sensors,6,7 water splitting,8,9 and so forth. In dye-sensitized solar cells, the high surface area enables large dye loading while the highly ordered configuration facilitates electron transport,10 thus rendering great potential for further efficiency enhancement. However, these unique properties can only be capitalized when the nanotube growth process is well studied for better geometry control. In the past decade, the length of anodic titania nanotubes increased from the initial hundreds of nanometers to a few *To whom correspondence should be addressed. E-mail: MSYZhang@ ntu.edu.sg. Telephone: þ65 67904400. Fax: þ65 67924062.
(1) Zwilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1999, 45, 921. (2) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (3) Varghese, O. K.; Paulose, M.; Grimes, C. A. Nat. Nanotechnol. 2009, 4, 592. (4) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (5) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (6) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Adv. Mater. 2003, 15, 624. (7) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Ong, K. G. Nanotechnology 2006, 17, 398. (8) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. (9) John, S. E.; Mohapatra, S. K.; Misra, M. Langmuir 2009, 25, 8240. (10) Sun, L.; Zhang, S.; Sun, X.; He, X. J. Nanosci. Nanotechnol. 2010, 10, 4551. (11) Beranek, R.; Hildebrand, H.; Schmuki, P. Electrochem. Solid-State Lett. 2003, 6, B12. (12) Cai, Q.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230. (13) Macak, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100. (14) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179. (15) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (16) Albu, S. P.; Ghicov, A.; Macak, J. M.; Schmuki, P. Phys. Status Solidi RRL 2007, 1, R65.
18424 DOI: 10.1021/la103390s
micrometers,2,11-13 and to hundreds of micrometers14-17 and even millimeters.18 This huge progress was achieved by adopting different anodizing electrolyte to reduce chemical dissolution of the fashioned nanotubes, that is, from the initial acidic electrolyte to mild aqueous electrolyte, and to organic electrolyte. In the case of organic electrolyte, electric field originating from potential drop in the electrolyte played an important part in the nanotube growth, which was discussed qualitatively using an ion flux model in our previous work.19 The pore diameter of the nanotubes was reported to be influenced by a number of factors in organic electrolyte, for example, applied potential,20 working distance,19 water content,21 and temperature,22 which was more than two times smaller than that in aqueous electrolyte.23 All these were also closely related to the potential drop in the electrolyte. Nevertheless, the essential role of this electric field was still ambiguous. In the current work, the effect of the electric field was directly revealed for the first time using a simple two-electrode anodizing method. Anodic titania nanotube arrays were grown simultaneously at both sides of a titanium foil. The tube length as well as the surface porosity increased with applied potential at each side, while longer nanotubes were obtained at the front side in comparison to the back side. This lopsided growth was attributed to the higher ionic flux induced by electric field at the front side. Accordingly, the double-sided nanotubes were tailored to be comparable by adjusting the electric field in the electrolyte. The tube growth rate under different anodizing configurations was also compared. It shows that the fastest growth was obtained with this simple two-electrode method. (17) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 7235. (18) Paulose, M.; Prakasam, H. E.; Varghese, O. K.; Peng, L.; Popat, K. C.; Mor, G. K.; Desai, T. A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 14992. (19) Sun, L.; Zhang, S.; Sun, X. W.; He, X. J. Electroanal. Chem. 2009, 637, 6. (20) Paramasivam, I.; Macak, J. M.; Selvam, T.; Schmuki, P. Electrochim. Acta 2008, 54, 643. (21) Macak, J. M.; Hildebrand, H.; Marten-Jahns, U.; Schmuki, P. J. Electroanal. Chem. 2008, 621, 254. (22) Macak, J. M.; Schmuki, P. Electrochim. Acta 2006, 52, 1258. (23) Macak, J. M.; Tsuchiya, H.; Taveira, L.; Aldabergerova, S.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 7463.
Published on Web 11/04/2010
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Figure 1. (a) Schematic diagram of a two-electrode electrochemical anodization. (b) Optical microscope image of the titanium foil in (a) anodized at 60 V for 30 min. FESEM cross-sectional images of the nanotube arrays prepared in (a) at 25 V for 30 min: (c) front side and (d) back side.
Experimental Section Prior to anodization, titanium foils (0.25 mm, 99.7% purity, Sigma-Aldrich) were ultrasonically cleaned in acetone, ethanol, and deionized water for 10 min each. The anodizing electrolyte solution used was ethylene glycol (anhydrous, 99.8%, Sigma-Aldrich) with 0.3 wt % ammonium fluoride (NH4F, 98þ%, ACS reagent, SigmaAldrich) and 2 vol % deionized water. All the experiments were carried out at room temperature (approximately 20 °C), unless otherwise highlighted, without stirring. A Keithley 2400 SourceMeter was employed as the power supply for the anodization and to record current transients during the process. Target potentials were directly applied onto the electrodes without ramping. Samples after anodization were rinsed with deionized water and dried in air. The morphology of the nanotubes was characterized by field emission scanning electron microscopy (FESEM, JEOL, JSM6340F) at an acceleration voltage of 5.0 kV and working distance of 7-10 mm. Some cross-sectional views with low magnification were obtained by scanning electron microscopy (SEM, JEOL, JSM-5910LV) at an acceleration voltage of 15.0 kV and working distance of 16 mm. The electrolyte conductivity was measured at an electrochemical workstation (Autolab, PGSTAT30). To examine the cross section by optical microscope, the nanotubes were first mounted in epoxy resin, ground with silicon carbide abrasive paper, and then polished with alumina suspension (BUEHLER, 0.05 μm) for a scratch free mirror surface. Surface porosity of the nanotubes was determined using OriginPro 8 software based on a method presented in the Supporting Information.
Results and Discussion Single-Sided Anodization for Double-Sided Nanotube Growth. For growth of double-sided nanotube arrays, a typical two-electrode configuration was employed, where a titanium foil was used as working electrode and a piece of platinum gauze as counter electrode. The back side of the titanium foil was exposed to the electrolyte, as shown in Figure 1a. In the same electrochemical system, the nanotube arrays grown at the front side (i.e., the side facing the counter electrode) have been investigated extensively, but the back side seems always ignored. Figure 1b is a crosssectional view of the titanium foil anodized at 60 V for 30 min. Titanium oxide layers exist at both sides of the foil, and the layer at the front side is thicker than that at the back side. FESEM characterization reveals that both the layers are highly ordered nanotubes, as shown in Figures 1c and d, with the tube length at the front side being longer than that at the back side. Langmuir 2010, 26(23), 18424–18429
Figure 2. FESEM top-view images of the nanotubes grown at 25 V (a, b), 40 V (c, d), 50 V (e, f), and 60 V (g, h) for 30 min. The left and right columns are nanotubes at the front side and back side, respectively. Insets are local magnifications or corresponding SEM crosssectional views.
The surface morphology of the nanotubes grown at different applied potentials is shown in Figure 2. Obviously all the nanotubes are covered by a thin layer of porous oxides that formed at the very beginning of anodization, because no significant chemical dissolution takes place during such a short duration (i.e., 30 min). Even so, the pore size as well as the tube length at both sides increases with applied potential. With prolonged anodizing duration19 or increased temperature, the tubes become clearer as a result of enhanced dissolution at the top surface (as displayed in Figure S1 in the Supporting Information). This will affect the final length of the nanotubes and thus the net growth rate. As such, a series of experiments was carried out at room temperature for 30 min only to study the lopsided growth process. The lengths of nanotubes prepared at different applied potentials are summarized in Figure 3a. The tube length increases with the applied potential at each side, with the front side being longer than the back side at all the applied potentials studied. This difference in tube length is attributed to different ionic fluxes in the electrolyte between the front side and back side, which will be discussed in detail in the following sections. Identification of Growth-Controlling Process during Nanotube Formation. Yasuda and Schmuki24 demonstrated that ionic diffusion in the electrolyte was the rate-determining step of zirconium titanate nanotube growth in aqueous electrolyte. In the case of titania nanotubes grown in organic electrolyte, the influence of electric field should also be taken into account, as presented in our previous work.19 To interpret the lopsided growth and reveal the role of the electric field, two major diffusion processes are considered, that is, Hþ ion diffusion toward the counter electrode and F- ion diffusion toward the working electrode. In anodic growth of titania nanotube arrays, the following chemical reactions take place at the working electrode:12,13,25,26 Ti þ 2H2 O=2OH- f TiO2 þ 4Hþ =2Hþ þ 4e
ð1Þ
TiO2 þ 6F- þ 4Hþ f TiF6 2- þ 2H2 O
ð2Þ
(24) Yasuda, K.; Schmuki, P. Electrochim. Acta 2007, 52, 4053. (25) Yasuda, K.; Macak, J. M.; Berger, S.; Ghicov, A.; Schmuki, P. J. Electrochem. Soc. 2007, 154, C472. (26) Kunze, J.; Seyeux, A.; Schmuki, P. Electrochem. Solid-State Lett. 2008, 11, K11.
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Figure 3. (a) Titania nanotube length as a function of applied potential for both front side and back side (data are from Figures 1 and 2). The anodizing duration is 30 min for all the experiments. (b) Current transients during nanotube growth at different applied potentials. Inset is a local region.
Reaction 1 is responsible for field-assisted oxidation, while reaction 2 contributes to field-assisted dissolution. Nanotube growth is an adjustment between these two processes. Essentially, the fluorine ion is a crucial factor in the formation of nanotubes, in the absence of which only a compact titanium oxide layer is formed through reaction 1.21,27 In the presence of fluorine ions, field-assisted dissolution takes place provided that enough Hþ and F- ions are present at the oxide surface simultaneously. Otherwise, the growth process is controlled by either field-assisted oxidation or field-assisted dissolution, depending on whether For Hþ ions are abundant at the oxide surface. For nanotube growth in the present organic electrolyte, the required Hþ ions originate mainly from field-assisted oxidation through reaction 1.13 Gas emission is observed from both titanium and platinum surfaces in the first tens of seconds after commencement of the electrochemical process. This suggests the existence of the following reactions under DC condition: 2H2 O f O2 þ 4Hþ þ 4e ðtitanium electrodeÞ 2H2 O þ 2e f H2 þ 2OH-
ðplatinum electrodeÞ
ð3Þ ð4Þ
(27) Bauer, S.; Kleber, S.; Schmuki, P. Electrochem. Commun. 2006, 8, 1321.
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Figure 4. (a) Schematic diagram of evolution of Hþ (green dots) and F- (red dots) ions during nanotube growth. FA-Dis and FAOx denote field-assisted dissolution and field-assisted oxidation, respectively. (b) (Bottom) nanotube length as a function of anodizing duration at 60 V for both the front side and back side. (Top) corresponding length difference between front side and back side.
Figure 3b shows the current transients during nanotube growth at different applied potentials. It shows the current density drastically decreases in the initial tens of seconds, along with the observation indicating that field-assisted oxidation is dominant at this stage. This results in a high Hþ concentration at the oxide surface through reactions 1 and 3. On the other hand, the required F- ions for tube growth are from the bulk electrolyte by diffusion process. In view of these, the initial stage of nanotube growth is controlled by F- flux toward the working electrode (i.e., the field-assisted dissolution process or reaction 2) as a result of abundant Hþ ions at the oxide surface, as illustrated in Figure 4a (see stage I). As the process progresses, the F- ions are continuously transported toward the oxide surface. As such, the Hþ ions are gradually consumed through reaction 2 until the concentration drops to a steady value, thus resulting in progressively increased tube length (stage II in Figure 4a). This steady value is the Hþ concentration produced only through reaction 1. After reaching the steady Hþ concentration, F- concentration increases with anodizing duration because of enhanced surface porosity. At a particular moment (stage III in Figure 4a), a critical ratio between Hþ and F- concentration is achieved, that is, 2:3 according to reaction 2. Thereafter, the nanotube growth will be controlled by Hþ generation (i.e., the field-assisted oxidation process or reaction 1), owing to the abundance of F- ions at the oxide surface (stage IV in Figure 4a). Langmuir 2010, 26(23), 18424–18429
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Existence of this transition process, that is, from field-assisted dissolution control to field-assisted oxidation control, is reflected in Figure 4b. It reveals that nanotube length at both sides increases with anodizing duration. Importantly, there is an abrupt increase in the growth rate at about 2 h. This indicates that the nanotube growth process is controlled by field-assisted dissolution before 2 h and by field-assisted oxidation after 2 h, as marked in Figure 4b. However, the growth rate will eventually decrease again for long anodizing durations, which will be discussed later. The Role of Electric Field in Nanotube Growth. Now let us return to Figure 3a. The tube length at the front side is longer than that at the back side at all applied potentials. Since the anodizing duration is 30 min for all these experiments, the tube growth process is controlled by field-assisted dissolution or exactly the Fflux toward the working electrode. Therefore, the higher the Fflux, the longer the tubes. As such, the longer tubes at the front side originate from the larger F- flux. Considering potential drop in the organic electrolyte, ionic flux at the front side should consist of two parts: ion diffusion under concentration gradient and ion migration under electric field. In contrast, only ion diffusion exists at the back side, as follows: Jf ¼ - pD
Dc þ ucE Dx
ð5Þ
Dc Dx
ð6Þ
Jb ¼ - pD
where Jf and Jb are ionic fluxes at the front side and back side, respectively, p is the porosity of the nanotube surface layer, D is the diffusion coefficient, c is the concentration, ∂c/∂x is the concentration gradient, u is the ion mobility, and E is the electric field strength in the electrolyte. In theory, mass transport in solution occurs by diffusion, migration, or convection. In this study, all the experiments were carried out under near constant ambient temperature (∼20 °C) for 30 min only (in this case, possible natural convection due to thermal gradients was negligible) without stirring (that is, no forced convection); therefore, the effect of convection can be neglected. Normally, the following conditions are chosen to minimize the migration effect in an electrochemical cell:28 using a large quantity (>0.1 M) of inert electrolyte (or supporting electrolyte), reducing electroactive species to a concentration of 5 mM or less, minimizing the distance between working electrode and counter electrode, employing electrodes of small sizes (e.g., microelectrodes), and so forth. Accordingly, the migration effect must be considered in the present study in view of the conditions used: in the absence of inert electrolyte (i.e., ethylene glycol þ 0.3 wt % NH4F þ 2 vol % deionized water, a solution volume of 77 mL in a rectangular vessel of 50 mm 60 mm 50 mm), large concentration of electroactive species (i.e., 0.3 wt %, or 0.09 M for NH4F), considerable distance between the electrodes (i.e., 13 mm), and large sizes of the planar electrodes (i.e., ∼11 mm 20 mm on average for titanium foils and 15 mm 25 mm for the platinum mesh). For the terms of ion diffusion in eqs 5 and 6, a factor of surface porosity p is involved to illustrate its impact on the flux. The surface porosity of the nanotubes (as displayed in Figure 2) is evaluated using a method presented in the Supporting Information. Figure 5a reveals that the porosity linearly increases with the applied potential at each side and is comparable under the same condition for both sides. Under pure diffusion control at the back side, the plot of tube (28) Brett, C. M. A.; Brett, A. M. O. Electrochemistry: principles, methods, and applications; Oxford University Press: New York, 1993; Chapter 5.
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Figure 5. (a) Potential dependence of surface porosity at front side and back side. Data are from Figure 2. (b) Plot of nanotube length versus surface porosity at the back side. Data are from Figures 3a and 5a.
length against porosity exhibits a proportional relationship, as shown in Figure 5b. In consideration of eq 6, this gives rise to LµJ
ð7Þ
Herein, L is the nanotube length. Consequently, it is the applied potential that affects the surface porosity and thus the ionic flux and eventually the tube growth rate. The potential dependence of ionic flux is also reflected in the inset of Figure 3b. In the first tens of seconds, the current density decreases drastically. At a particular moment, a turning point appears, indicating occurrence of obvious dissolution (i.e., formation of pore embryos). This turning point (marked with arrows in Figure 3b) presents earlier at relatively higher applied potential, as a consequence of higher ionic flux. In eq 5, an additional term of ion migration is produced owing to the potential drop in organic electrolyte at the front side. This potential drop (U) can be estimated using Ohm’s Law: 1l l ¼ i ð8Þ KA K where I is the current flow, R is the electrolyte resistance between the working and counter electrode, κ is the ion conductivity in the electrolyte, l is the distance between the two electrodes, A is the effective surface area of the working electrode, and i is the relevant current density. Note that herein the current density is computed U ¼ IR ¼ I
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using the one-side surface area only (i.e., A instead of 2A). Accordingly, the pertinent electric field strength in the electrolyte can be written as E ¼
U i ¼ l K
ð9Þ
Based on eqs 5-9 and the comparable surface porosity at the front side and back side (cf. Figure 5a), the following relationship stands: ΔL µ ΔJ µ E
ð10Þ
where ΔL and ΔJ are the differences in length and F- flux between front side and back side, respectively. As such, ΔL is directly proportional to the electric field strength E. In Figure 6, a plot of ΔL versus E is presented. It is evident that the nanotube length difference increases linearly with the electric field strength, in line with the above discussion. More important, this straight line passes through the origin. This predicts that the length difference will eventually disappear as E approaches zero. Equation 9 shows the value of E is proportional to current density i and inversely proportional to conductivity κ. The conductivity of the organic solution used is about 460 μS cm-1,17 which is small enough to magnify the electric field strength, thus giving rise to distinctly different tube lengths at the front side and back side. To demonstrate the effect of the electric
Figure 6. Plot of difference in nanotube length between front side and back side versus electric field strength in the electrolyte. Data are from Figure 3. The value of conductivity (κ) used is 460 μS cm-1.17
field, anodization was carried out in another electrolyte of much higher conductivity (i.e., 1 M (NH4)2SO4 þ 0.5 wt % NH4F, κ ≈ 4.45 104 μS cm-1, about 100 times larger than the normal electrolyte) also at 25 V for 30 min. The obtained nanotube arrays are shown in Figure 7, which are not as regular as those in the organic electrolyte (cf. Figures 1c and d) due to significant chemical dissolution. However, the bottoms of the nanotubes can be clearly discerned and remnant oxides at the surface can be observed as well. This morphology is in agreement with that reported by Taveira and co-workers.29 It is apparent that the length of the nanotubes is comparable at the front side and back side, identical to the above prediction and discussion. The results demonstrate that additional electric field at the front side contributes to ion migration in the electrolyte and gives rise to higher ionic flux, therefore resulting in longer nanotubes than those on the back side. In our previous work,19 anodic nanotube growth at steady state (i.e., the growth process controlled by field-assisted oxidation) was discussed in detail using an ion flux model. This model illustrated significant chemical dissolution of fashioned nanotubes kicked off much faster under low electric field, because of relatively smaller ionic flux in the electrolyte. Accordingly, in the current work, dissolution of the nanotubes at the back side should be triggered much earlier than that at the front side. This is confirmed in Figure 4b. It shows that the tube growth rate at the back side declines after approximately 4 h, and the corresponding length difference (top curve) keeps increasing thereafter, as a consequence of substantial chemical dissolution. Figure 8 is the pertinent surface morphology of the nanotubes (or nanowires30-34 on top of the nanotubes) at the front side and back side for 8 h anodization. It is evident that the remnant original oxide layer (porous surface) can still be observed at the front side, whereas this layer has totally disappeared at the back side. This directly reveals that chemical dissolution initiates much faster at the back side. On the other hand, it further proves that ionic flux at the back side is smaller than that at the front side, echoing the above discussion. Comparison between Single- and Double-Sided Anodization. In the normal anodizing process, as illustrated in Figure 1a (hereafter referred to as “SGunprot” for “single-sided anodization with titanium back side unprotected”), titania nanotubes develop simultaneously at both sides of a titanium substrate. A general question is what will happen to the front side if the back side growth is hindered or accelerated. As such, two more experiments were set up. In one case, the back side of the titanium foil was protected by epoxy resin (hereafter referred to as “SGprot” for “single-sided
Figure 7. FESEM cross-sectional images of as-grown titania nanotube arrays prepared at 25 V for 30 min in 1 M (NH4)2SO4 with 0.5 wt % NH4F: (a) front side and (b) back side. The arrows indicate the thickness of the nanotube layer. 18428 DOI: 10.1021/la103390s
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Figure 8. FESEM top-view images of as-grown titania nanotube arrays prepared at 60 V for 8 h: (a) front side and (b) back side.
Figure 9. Potential dependence of nanotube length at the front side under different back side environments. Data are from Figures 1 and 2, and Figure S2 in the Supporting Information. SGprot, SGunprot, and DB denote single-sided anodization with titanium back side protected, single-sided anodization with titanium back side unprotected, and double-sided anodization, respectively. The anodizing duration is 30 min for all the experiments.
anodization with titanium back side protected”). In yet another setup, two platinum counter electrodes were placed at each side of the titanium foil to establish a double-sided anodization (hereafter referred to as “DB” for “double-sided anodization”). Potential dependence of the tube length at the front side is given in Figure 9. It reveals that the tube length increases with applied potential regardless of conditions, due to relatively higher F- flux produced at higher potential. The pertinent growth rate presents the largest for SGunprot and the smallest for SGprot, with DB in between at nearly each applied potential. In the case of single-sided anodization including SGunprot and SGprot, there is one electronic pathway and one ionic pathway in (29) Taveira, L. V.; Macak, J. M.; Tsuchiya, H.; Dick, L. F. P.; Schmuki, P. J. Electrochem. Soc. 2005, 152, B405. (30) Lim, J. H.; Choi, J. Small 2007, 3, 1504. (31) Friedman, A. L.; Panaitescu, E.; Richter, C.; Menon, L. J. Nanosci. Nanotechnol. 2008, 8, 5864. (32) Song, Y.-Y.; Lynch, R.; Kim, D.; Roy, P.; Schmuki, P. Electrochem. SolidState Lett. 2009, 12, C17. (33) Wang, J.; Zhao, L.; Lin, V. S.-Y.; Lin, Z. J. Mater. Chem. 2009, 19, 3682. (34) Wu, Z.; Guo, S.; Wang, H.; Liu, Y. Electrochem. Commun. 2009, 11, 1692.
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the electrochemical system. During nanotube growth, electrons are generated at the working electrode through reaction 1 as well as reaction 3, and consumed at the counter electrode through reaction 4. In the SGunprot system, extra nanotube growth at the back side will produce more electrons in comparison to the SGprot system, resulting in relatively higher current flow. Since there is only one ionic pathway, the relevant current density at the front side will be increased accordingly. On the basis of eq 9, this increased current density will, in turn, promote the electric field in the electrolyte, thereby giving rise to higher ionic flux (cf. Equation 5) and hence the growth rate. With respect to double-sided anodization, there are two electronic pathways and two ionic pathways. In this system, nanotubes grown at the back side will no longer contribute to the current density of the front side. This is the reason why the growth rate at the front side, for the DB system, is smaller than that for the SGunprot system. As a consequence, the SGunprot system gives rise to the largest growth rate among all the conditions. In addition, it is believed that the relatively higher nanotube growth rate for the DB system originates from the higher ionic flux (or higher current density), due to relatively lower resistance in the parallel circuit in contrast to the SGprot system.
Conclusions Titania nanotube arrays are simultaneously grown at both sides of a titanium foil using a two-electrode anodizing method. The tube length and surface porosity at both sides increase with applied potential. Under normal conditions, longer nanotubes are obtained at the front side compared to the back side. This lopsided growth process is attributed to the higher ionic flux induced by electric field at the front side. The double-sided nanotubes are tailored to be comparable by adjusting the electric field in the electrolyte. Comparative study reveals this simple twoelectrode anodizing configuration gives rise to the largest nanotube growth rate. Use of this double-sided nanotube structure promises high efficiency in parallel configuration dye-sensitized solar cells, which is currently under investigation in our group. Acknowledgment. Part of the funding for this work comes from a collaboration project M48050127. Supporting Information Available: FESEM surface and cross-sectional views of the nanotubes; determination of surface porosity based on electron microscopy image. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la103390s
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